Hydraulic radial piston motor

ABSTRACT

A hydraulic motor having radial pistons, and comprising a cam, a cylinder block and a distributor. The cylinders of the cylinder block are connected to communication orifices ( 32 A) situated in a communication face of the cylinder block, while the distributor has a distribution face in which distribution orifices ( 21 A,  23 A) are provided that are suitable for communicating with the communication orifices for rotating the motor. The cam is provided with a plurality of lobes ( 50, 50′ ), each of which has two ramps ( 51, 52 ), each ramp corresponding to a distribution orifice ( 21 A,  23 A). The edge of each of at least certain communication orifices ( 32 A) has at least one notch ( 54 A,  54 B) suitable for establishing a small section of communication with a distribution orifice. The invention applies in particular to hydraulic motors having radial pistons for which the ratio between the number of cylinders and the number of cam lobes is in the vicinity of 1:

The present invention relates to a hydraulic motor having radialpistons, and comprising a cam and a cylinder block that are suitable forturning relative to each other about an axis of rotation, the cylinderblock having radial cylinders connected via cylinder ducts tocommunication orifices situated in a communication face of the cylinderblock that is perpendicular to the axis of rotation, pistons mounted toslide in the cylinders being suitable for co-operating with the cam,which cam is provided with a plurality of lobes, each of which has tworamps, the motor further comprising a fluid distributor having adistribution face that is perpendicular to the axis of rotation and thatis suitable for being in abutment against the communication face of thecylinder block, said distribution face being provided with distributionorifices comprising orifices suitable for being connected to a fluidfeed and orifices suitable for being connected to a fluid discharge, thefluid distributor being constrained to rotate with the cam, so thatthere is one ramp of the cam that corresponds to each distributionorifice, said distribution orifices being suitable for communicating oneafter another with the communication orifices while the cylinder blockand the distributor are turning relative to each other.

For a motor of this type, operating at full cubic capacity, eachcommunication orifice comes successively to face a distribution orificeconnected to the fluid feed and comes to face a distribution orificeconnected to the fluid discharge. The communication orifice in questionbeing coupled to the distribution orifice that is connected to the feedcauses the piston contained in the cylinder connected to saidcommunication orifice to be pushed radially outwards, while the samecommunication orifice being coupled to a distribution orifice that isconnected to the fluid discharge makes it possible to cause said pistonto return into its cylinder, towards the axis of the motor. Thus, eachpiston co-operates successively with the various portions of the lobesof the cam so as to enable the cylinder block and the cam to rotaterelative to each other.

The spacing between the distribution orifices and the spacing being thecommunication orifices are such that a communication orifice is notsimultaneously connected to two distribution orifices respectivelyconnected to the fluid feed and to the fluid discharge.

While the cylinder block and the distributor are rotating relative toeach other, the working chambers of the cylinders, i.e. the portions ofsaid cylinders that are defined under the pistons, are placedalternately at high pressure and at low pressure. Therefore in saidworking chambers, changes in pressure generally take place at a veryfast rate. Such changes in pressure subject the pistons to proportionalforces, and said forces are transmitted by the pistons to the cam.

As a result, the components of the motor, in particular its casing, aresubjected to the variation in load, which causes noise-generatingvibration, the intensity of the noise generated depending mainly on thespeed of the increases and decreases in pressure in the workingchambers.

In order for the motor to operate correctly, the difference in pressurebetween the fluid feed and the fluid discharge is large. When a pistoncontributing to the drive torque reaches the end of its stroke towardsits position that is furthest from the axis of the motor (top deadcenter), due to the communication orifice of its cylinder beingconnected to a distribution orifice connected to the fluid feed, thesame communication orifice is isolated from said distribution orifice,and is then connected to another distribution orifice which is connectedto the fluid discharge. This results in a phenomenon of pressurereduction in the cylinder of the piston in question, the fluid presentat a high pressure in the cylinder being suddenly put into communicationwith a significantly lower pressure, which is the pressure of the fluiddischarge. Conversely, when the piston reaches the bottom dead center ofits stroke (its position that is closest to the axis of the motor), itscylinder is isolated from the fluid discharge, and is then connected tothe fluid feed so as to enable the piston to travel over a centripetalstroke again. At this instant, the fluid contained in the cylinder goesfrom a low pressure to a pressure that is much higher, which is thepressure of the fluid feed. A phenomenon of pressure reduction alsogenerally takes place, from the fluid feed, towards the cylinder. In thepreceding case, the pressure reduction takes place from the cylindertowards the fluid discharge.

In both cases, the pressure reductions that take place generate joltingor juddering sensations, and noises such as knocking.

The more the quality of such motors is improved, and the greater theextent to which leaks are reduced in such motors, the more perceptiblesuch phenomena become. In old motors, the leaks prevailing in them madeit possible to avoid variations in pressure that were too sudden betweenthe various enclosures.

In order to avoid, or at least to limit the jolting phenomena due, inparticular, to the fluid contained in the working chambers of thecylinders expanding too quickly when the communication ducts are putinto communication, via the distribution orifices, with the fluid feed,it is possible to equip the edge of each of the distribution orificeswith at least one notch suitable for acting, while the cylinder blockand the distributor are rotating relative to each other, to establish asmall section of communication between the working chambers of thecylinders and the distribution ducts. The small section ofcommunication, which is open for a very short time, makes it possible toavoid pressure variations that are too sudden in the working chambers.

However, the applicant has observed that such a solution is not alwayseasy to implement. In certain cases, it is not very easy and/or it iscostly to form notches in the edges of the distribution orifices.

Therefore, an object of the invention is to propose another solution foravoiding or limiting the above-mentioned jolting phenomena.

This object is achieved by the fact that the edge of each of at leastcertain communication orifices is provided with at least one notchsuitable for establishing a small section of communication with adistribution orifice.

The invention can be used particularly advantageously for motors inwhich the ratio of the number of cylinders to the number of cam lobes isin the vicinity of 1. A hydraulic motor having radial pistons has onedistribution orifice for each of the ramps of its cam lobes, i.e. it hastwice as many distribution orifices as it has cam lobes. Conversely, themotor has one communication orifice for each cylinder. Therefore, whenthe above-mentioned ratio is in the vicinity of 1, the motor has abouttwice as many distribution orifices as it has communication orifices.Forming notches in the edges of the communication orifices is thusconsiderably less costly than forming notches in the edges of thedistribution orifices in the same type of motor.

As explained below, the invention covers the case when the edge of eachcommunication orifice is provided with at least one notch, and also thecase when only some of said orifices have their edges provided withnotches, each of those orifices being provided with one or more notches.

Having said that, in either case, for the same technical effect and fora motor in which the ratio of the number of cylinders to the number ofcam lobes is in the vicinity of 1, the number of communication orificesthat need to be provided with notches is significantly smaller than thenumber of distribution orifices that need to be provided with notches.

The dimensions of the notches are chosen to allow a volume of fluid topass gradually between the orifices at different pressures through saidnotches while the cylinder block and the fluid distributor are rotatingrelative to each other, which volume is referred to as a“pressure-compensating” volume, corresponding, for given operatingrotation speeds and pressures, to decompression or pressure reduction ofthe maximum volume of the working chamber that is obtained at the topdead center of the piston on the cam. Passing a pressure-compensatingvolume of fluid through the constriction constituted by a notch, beforefully-fledged communication is established between the communicationorifice and a distribution orifice makes it possible to cause thepressure of the fluid at the communication orifice to vary gradually tobring it gradually to the pressure of the fluid at the distributionorifice. The lapse of time for which the notch makes it possible forfluid to pass between the communication orifice and the distributionorifice while the cylinder block and the distributor are rotatingdepends on the speed of rotation of the motor. That is why the operatingpressures and the rotation speed are parameters to be taken into accountin defining the notch.

The edge of each communication orifice has a leading portion via whichcommunication between the communication orifice and the distributionorifices opens while the cylinder block and the distributor are turningrelative to each other in a given direction of relative rotation, and atrailing portion via which communication between the communicationorifice and the distribution orifices closes while the cylinder blockand the distributor are turning relative to each other in the samedirection of relative rotation.

In an advantageous variant, particularly suitable for motors having twooperating directions, each leading portion and each trailing portion ofthe edges of at least certain communication orifices has a notchsuitable for establishing a small section of communication with adistribution orifice.

The applicant has observed that the notches are particularly useful whenthe communication between the distribution orifices and thecommunication orifices is opening because, at that time, the pressure ofthe fluid contained in the working chambers and the pressure of thefluid contained in the distribution ducts (be it the feed pressure orthe discharge pressure) are significantly different, and it is thispressure difference that, if the distribution orifices open toosuddenly, generates jolts and noise. In other words, the presence of thenotches is particularly desirable in the leading portions of thecommunication orifices. In certain uses, the motor always or nearlyalways turns in the same direction of rotation. This applies, forexample, when the motor serves to drive grinding mills, conveyor beltsor concrete mixers. In which case, it can suffice for only thoseportions of the edges of the communication orifices which, in thisdirection of rotation, form the leading portions of said edges, to carrynotches.

In other uses, the motor is reversible, with two opposite directions ofrotation. This applies, for example, when it serves to drive a turret ofa mechanical digger. In which case, two opposite portions of the edgesof each of the communication orifices can, depending on the direction ofrotation of the motor, be a leading portion or a trailing portion. It istherefore desirable for each of the two portions to be provided with anotch.

In numerous uses, the motors are reversible and do not have a preferreddirection of operation. For example, this applies when driving certaintypes of vehicles in translation, in particular tracked vehicles.

In which case, advantageously, the notches in the leading portion and inthe trailing portion of the edge of each of said communication orificesare symmetrical.

Certain reversible motors do have a preferred operating direction. Forexample, motors for driving vehicles in translation can operate mainlyat high speed in the forward direction while their speed is limited inthe reverse direction. In which case, it is possible to provide largenotches in those portions of the edges of the communication ducts whichare leading portions in the preferred direction, and small notches canbe provided on the portions opposite from said edges, which portions arethe trailing portions in said preferred direction and the leadingportions in the opposite, non-preferred direction.

The large notches make it possible to allow sections of communicationwith the distribution orifices that are larger than the sections ofcommunication allowed by the small notches, e.g. in proportion to theratio between the highest speeds of rotation of the motor permittedrespectively in the preferred direction and in the non-preferreddirection of said motor.

In an advantageous embodiment, in which each ramp of the cam has aconvex portion and a concave portion, two adjacent ramps being connectedtogether either via a cam crest zone extending between their respectiveconvex regions, or via a cam trough zone extending between theirrespective concave regions, said cam crest zone and said cam trough zoneare substantially circular arcs centered on the axis of rotation, sothat when the pistons are co-operating with said zones, their radialstrokes are substantially zero. The distribution orifices and thecommunication orifices thus advantageously have dimensions such that,while the cylinder block and the distributor are rotating relative toeach other, each distribution orifice remains momentarily isolated fromany communication orifice.

The cam crest and cam trough zones are substantially circular arcscentered on the axis of rotation, which means that the radii ofcurvature of said zones, as measured between their ends, are, for thecam crest zones, substantially equal to the minimum radial distance fromthe cam to the axis of rotation and, for the cam trough zones,substantially equal to the maximum radial distance from the cam to theaxis of rotation. The radius of curvature of each of said zones can,however, be different respectively from the minimum radius and from themaximum radius of the cam but, substantially, their distances to theaxis of the motor are respectively equal to said minimum and maximumradii. When a piston is co-operating with such zones, its radial strokeis substantially zero, which means that said stroke is zero or is at themost about 0.5% of the maximum amplitude of the stroke of the piston.Thus, the cam crest zones and the cam trough zones do not contribute tothe drive torque. They cover small angular sectors, e.g. about 2° to 3°,and, while the cylinder block and the distributor are rotating relativeto each other, they make it possible to offer dead center instants foreach piston (top dead center for the cam trough zones and bottom deadcenter for the cam crest zones), during which the pressure in theworking chamber of the cylinder in which the piston in question ismoving can, by means of the compensation volume of fluid passing througha notch, equal or come close to the pressure of the distributionorifice.

It is particularly advantageous for the lapse of time for which a givencommunication orifice is in communication with a distribution orificesolely via the notch that is provided in the edge of said communicationorifice to lie within the lapse of time for which the piston fed viasaid communication orifice co-operates either with a cam crest zone orwith a cam trough zone. It is advantageous to use this time for whichthe piston does not develop any torque to cause the pressure in theworking chamber to vary gradually by means of the notch in the edge ofthe communication orifice.

Advantageously, the angular sectors covered by a cam crest zone and by acam trough zone are substantially equal to each other and liesubstantially in the range 2° to 3°.

The invention will be well understood and its advantages will appearmore clearly on reading the following detailed description of anembodiment shown by way of non-limiting example.

The description refers to the accompanying drawings, in which:

FIG. 1 is an axial section view of a hydraulic motor to which theinvention can be applied;

FIG. 2 is a fragmentary radial section view on line II-II of FIG. 1;

FIG. 3 is a view in section on the circular arc III-III of FIG. 2; and

FIGS. 4 and 5 show, in fragmentary radial section, two variantembodiments.

FIG. 1 shows a hydraulic motor comprising a fixed casing in threeportions 2A, 2B, and 2C, assembled together by bolts 3.

Naturally, the invention is not limited to hydraulic motors having fixedcasings, but rather it is also applicable to hydraulic motors havingrotary casings and that are well known to the person skilled in the art.

The portion 2C of the casing is closed axially by a radial plate 2D thatis also fixed by bolts. An undulating reaction cam 4 is formed on theportion 2B of the casing.

The motor includes a cylinder block 6 which is mounted to rotate aboutan axis of rotation 10 relative to the cam 4, and which comprises aplurality of radial cylinders which are suitable for being fed withfluid under pressure, and inside which the radial pistons 14 areslidably mounted.

The cylinder block 6 rotates a shaft 5 which co-operates with it viafluting 7. The shaft carries an outlet flange 9.

The motor also includes an internal fluid distributor 16 which issecured to the casing so that it is prevented from rotating relativethereto about the axis 10. Between the distributor 16 and the insideaxial face of the portion 2C of the casing, distribution grooves areformed, namely a first groove 18, a second groove 19, and a third groove20. The distribution ducts of the distributor 16 are organized in afirst group of ducts which, like the duct 21, are all connected to thegroove 18, a second group of ducts (not shown) which are connected tothe groove 19, and a third group of ducts which, like the duct 22, areconnected to the groove 20. The first groove 18 is connected to a firstmain duct 24 to which all of the distribution orifices of thedistribution ducts of the first group, such as the orifice 21A, areconnected. The third groove 20 is connected to a second main duct 26 towhich all of the distribution orifices of the ducts of the third group,such as the orifice 22A of the duct 22, are connected.

Depending on the direction of rotation of the motor, the main ducts 24and 26 are respectively a fluid exhaust duct and a fluid feed duct, orvice versa.

The distribution ducts open out in a distribution face 28 of thedistributor 16, which face is in abutment against a communication face30 of the cylinder block. Each cylinder 12 has a cylinder duct 32 thatopens out in said communication face so that, while the cylinder blockand the cam are rotating relative to each other, the cylinder ducts comeinto communication in alternation with the distribution ducts of thevarious groups.

The motor of FIG. 1 also includes a cubic capacity selector devicewhich, in this example, comprises a bore 40 that extends axially in theportion 2C of the casing and in which an axially-movable selector slide42 is disposed. The bore 40 is provided with three communication ports,respectively 44, 46, and 48, which are connected to respective ones ofthe grooves 18, 19, and 20, via connection ducts, respectively 44′, 46′,and 48′. The slide 42 is mounted to move between two end positionsinside the bore 40, in which positions it causes the ports 44 and 46 orthe ports 46 and 48 to communicate via its groove 43.

When the selector 42 is in its position shown in FIG. 1, the grooves 19and 20 communicate, so that the distribution orifices that are connectedto them are at the same pressure, which is different from the pressureof the distribution orifices connected to the groove 18. When theselector 42 is moved in the direction indicated by arrow F, it is thedistribution orifices connected to the grooves 18 and 19 that are put atthe same pressure, which is different from the pressure at which theorifices connected to the groove 20 are put.

FIG. 2 shows a cam lobe with its two ramps, respectively 50 and 50′.Each of the two ramps has a convex portion, respectively 51 and 51′, anda concave portion, respectively 52 and 52′. The convex portions are theportions that are closer to the axis of rotation 10 of the motor, whilethe concave portions are the portions that are further away from saidaxis. A piston 14 co-operates with the cam crest zone 58, via which theconcave portions 52 and 52′ of the ramps 50 and 50′ meet. Said piston isin its top dead center position, i.e. the volume of the working chamberof the cylinder in which it moves is at its maximum. Other pistons 14′and 14″ co-operate with other zones of the cam.

At this time, for the reasons described below, the communication orifice32A via which the cylinder in which the piston moves 14 can be fed withfluid under pressure, and via which the fluid contained in the cylindercan be discharged, is isolated from any distribution orifice.

For reasons of clarity, FIG. 2 shows two distribution orifices,respectively 21A and 23A, e.g. connected to respective ones of thegrooves 18 and 19, although said grooves are not normally visible in thesection view. The positions of two other communication orifices, namely32′A and 32″A, are also indicated.

The two communication orifices of FIG. 2 are identical, and each of themis provided with two notches, respectively 54A and 54B, suitable forestablishing a small section of communication between the communicationorifice equipped with said notches and the distribution orifices.

If it is considered that the cylinder block is moving relative to thedistributor in the direction of rotation R1, the portion B1 of the edgeof the communication orifice 32A in which the notch 54A is formed is aleading portion, i.e. it is via this portion that communication opensbetween the communication orifice 32A and the distribution orifice 21A.Thus, in a first stage, the communication is established via the notch54A only, over an angle of relative rotation of the distributor and ofthe cylinder block equal to the angular amplitude α₁ covered by thenotch 54A. By means of the notch 54A, the communication between thecommunication orifice 32A and the distribution orifice 21A opensgradually, so that the pressure in the working chamber of the cylinderin which the piston 14 moves and the pressure in the distribution ductthat opens out at the distribution orifice 21A can be balancedgradually. When the rotation continues, the communication opens widelyas the orifices 32A and 21A mutually cover each other angularly.

When the cylinder block is rotating in the direction R1 relative to thedistributor, the portion B2 of the edge of the communication orifice 32Athat is opposite from the portion B1 constitutes a trailing portion, viawhich the communication between the communication orifice 32A and thedistribution orifice 23A closes.

When the direction of rotation of the motor is reversed so that thecylinder block rotates relative to the distributor in the direction ofrotation R2, it is the portion B2 of the edge of the communicationorifice that constitutes a leading portion. In which case, thecommunication between the communication orifice 32A and the distributionorifice 23A opens via the notch 54B. The pressures in the workingchamber of the cylinder in which the piston 14 moves and in thedistribution duct that opens out at the distribution orifice 23A canthen balance more gradually than in the prior art, in which thecommunication orifices are not provided with notches.

Thus, depending on the direction of rotation of the motor, one or otherof the notches 54A and 54B is useful for avoiding or at least forlimiting the phenomena of jolting when two enclosures in which differentfluid pressures prevail are put into communication with each other toorapidly.

In the example of FIG. 2, the notches 54A and 54B are symmetrical abouta diameter D of the communication orifice 32A that passes through theaxis of rotation 10 of the motor. This can be seen in FIG. 3. Asindicated above, it is however possible to make provision for thenotches not to be symmetrical. In particular if the direction ofrotation R1 in which the cylinder block is rotating relative to thedistributor corresponds to the preferred operating direction of themotor and if, in its non-preferred direction, the speed is lower thanthe speed that can be reached by the preferred direction, then the notch54A can be larger than the notch 54B.

The cam crest zone 56 and the cam trough zone 58 extend over respectiveangular sectors α₅₆ and α₅₈, as measured between two radii passingthrough the axis of the motor, that are substantially equal to eachother and that are approximately equal to in the range 2° to 3°.

Advantageously, in order to make advantageous use of the cam troughzones and of the cam crest zones for balancing the pressures between theworking chambers of the cylinders and the feed or discharge ducts of themotor, provision is made to choose the lapse of time for whichcommunication between a communication orifice and a distribution orificetakes place via a notch to lie within the lapse of time for which thepiston of the cylinder fed by said communication orifice co-operateswith a cam trough zone or with a cam crest zone. Times for which thepistons are not contributing to the drive torque are thus usedadvantageously for balancing the pressures.

For example, with an angular sector of sealing α₂ between the duct 21Aand the duct 32A, provision is made to choose α₅₈=2 (α₁+α₂), the camtrough zone being symmetrical about a radius R that determines an axisof symmetry for the cam lobe that has the ramps 50 and 50′; the angularsector of sealing α₂ and the angular coverage of a notch α₁ should beassessed, for a given direction of rotation, relative to a half camtrough zone. The same remark applies to the subject of the cam crestzone 56 which is symmetrical about the radius RS.

It is possible to make provision to chose the communication orifices tobe circular except for the presence of the notches. In which case, thenotches can be formed by moving a milling cutter in a diametrical planeof the communication orifice, which milling cutter cuts slightly intothe edges of said orifice. In order to form symmetrical orifices, adiameter of the milling cutter can be coaxial with the diameter of thecommunication orifice, while, to interconnect asymmetrical notches, thediameter of the milling cuter can be offset slightly relative to thediameter of the communication orifice.

In the example shown in FIG. 4, and like the orifice 32A, thecommunication orifice 132A has a leading portion B1 and a trailingportion B2 when the cylinder block turns relative to the distributor inthe direction of rotation R1. However, it can be noted that, for theorifice 132A, the leading portion B1 and the trailing portion B2 aresubstantially convex, as seen from inside the orifice. With theexception of the notches 154A and 154B, the leading portion and thetrailing portion form substantially circular arcs suitable for coveringthe edges of the distribution orifices 21A and 23A while the cylinderblock is rotating relative to the distributor. The shape of thecommunication orifice is substantially complementary to the shape of thedistribution orifices 21A and 23A.

If it is considered that the cylinder block is turning in the directionof rotation R1 relative to the distributor, then the communicationbetween the communication orifice 132A and the distribution orifice 21Astarts via the notch 154A which, as indicated above, establishes a smallsection of communication making it possible to balance gradually thepressures in the enclosures respectively connected to the distributionorifice and to the communication orifice. However, as soon as the angleof relative rotation between the cylinder block and the distributor issufficient, the leading portion B1 of the communication orifice passesbeyond the edge D1 of the distribution orifice, in the direction R1,and, as from this situation, the section of overlap between thecommunication orifice and the distribution orifice increases veryquickly as a function of the angle of relative rotation between thecylinder block and the distributor. In other words, as soon as thepressure in the enclosures connected respectively to the communicationorifice and to the distribution orifice have been more or less balancedvia the small section of communication allowed by the notch 154A, thecommunication between the orifices 132A and 21A can increase veryquickly, with very low head loss. By means of the notch, the joltingphenomena are avoided or at least limited and, by means of theparticular shape of the leading portion B1, the efficiency of the motoris increased.

If the motor has a single direction of rotation, in which the cylinderblock turns in the direction R1 relative to the distributor, then it isnot necessary for the trailing portion B2 to have a shape substantiallycomplementary to the shape of the portion D2 of the edge of the orifice23A via which the communication between the orifices 132A and 23Acloses. Conversely, if the motor has two operating directions, neitherof which is preferred, then the trailing portion B2, which becomes aleading portion in the direction of rotation R2, is advantageouslyshaped like the portion B1, symmetrically about a line L of symmetry ofthe orifice 132A that passes through the axis of rotation of the motor.

Naturally, when the motor has two directions of rotation, only one ofwhich is preferred, it is possible to choose to make provision for onlythose portions of the edges of the communication orifices whichconstitute leading portions in a preferred direction to be formedsubstantially complementary to those portions of the edges of thedistribution orifices via which the communication between thecommunication orifices and the distribution orifices open.

When the distribution orifices are circular, the communication orificescan, with the exception of the notches, have shapes of the typedescribed in FR-A-2 587 761. Conversely, the distribution orifices canhave a shape such that, without their notches, the communicationorifices are circular.

FIG. 5 shows another variant, in which the communication orifice 232Ahas a shape that is substantially elongate along a radius of the motorsthat passes through the axis of rotation thereof. In other words, exceptfor the notches 254A and 254B, the dimension of the orifice 232A, asmeasured along a radius of the motor, is larger than the dimension ofsaid orifice as measured transversely to said radius. Without having therelatively complex shape of the orifice 132A, the communication orifice232A offers, like that orifice, the advantage of enabling thecommunication between the communication orifice 232A and thedistribution orifice 21A or the distribution orifice 23A to open veryquickly as from the time when the pressure in the enclosures connectedrespectively to the communication orifice and to the distributionorifices has been substantially balanced due to the orifices in questionbeing put into limited communication via the notch 254A or via the notch254B.

FIG. 5 shows that the notch 254A is larger than the notch 254B, thedirection of rotation R1 in which the cylinder block rotates relative tothe distributor being a preferred direction relative to the oppositedirection R2.

In the figures described above, the notches in the communicationorifices are substantially disposed on a circular arc going through theaxis of rotation of the motor.

In a variant (not shown), it is also possible to choose to dispose allof the notches in the leading portions of the orifices on a first circlecentered on the axis of rotation of the motor, and all of the notches inthe trailing portions of said orifices on a second circle, of radiusdifferent from the first circle.

As indicated above, the motor shown in FIG. 1 has two active operatingcubic capacities, a cubic capacity selector making it possible to putcertain distribution ducts into communication with one another. Some ofthe pairs of consecutive communication orifices have orifices connectedto the same pressure, so as to make the motor operate in its small cubiccapacity.

Another manner exists for making the motor operate in two differentcubic capacities, consisting in making certain pistons inactive. Thattype of small cubic capacity control is described, for example, byPatent Application No. FR-A-2 796 992.

In which case, the deactivated pistons are, in general, declutched, bybeing brought towards the axis of rotation of the motor. In any event,in this situation, only the pistons that remain active contribute togenerating drive torque. In the small cubic capacity, for the sameflow-rate of fluid delivered by the pump feeding the motor, said motorturns at a speed higher than the speed at which it would be turned forthe same flow-rate of fluid in the large cubic capacity.

The above-mentioned pressure reduction and jolting phenomena are evenmore perceptible when the motor operates at high speed. Thus, in theinvention, it is possible to make provision for only each of thosecommunication orifices of the piston cylinders which are active in thesmall operating cubic capacity to be provided with at least one notch inits edge. In which case, each of the communication orifices of thosecylinders has a notch, while the communication orifices of the othercylinders are not provided with notches. Depending on whether or not themotor is reversible, and for the reasons indicated above, it is possibleto make provision for each of the communication orifices having at leastone notch to have a single notch or else to have two notches, servingfor opening the communication between said orifices and the distributionorifices in respective ones of the two operating directions of themotor.

In which case, since the maximum speed of rotation is smaller in thelarge cubic capacity than in the small cubic capacity, it can be deemedunnecessary to provide notches in the orifices of the cylinders of thepistons that are inactive in the small cubic capacity.

In a variant, it is possible to form the notches in the edges of all ofthe communication orifices. However, insofar as the maximum speed in thelarge cubic capacity is lower than the maximum speed in the small cubiccapacity, it is possible to make provision for the edges of thecommunication orifices of the cylinders whose pistons are active in thesmall cubic capacity to have notches that are larger than the notches inthe edges of the communication orifices of the cylinders of the pistonsthat are inactive in the small cubic capacity.

1-10. (canceled)
 11. A hydraulic motor having radial pistons, andcomprising a cam and a cylinder block that are suitable for turningrelative to each other about an axis of rotation, the cylinder blockhaving radial cylinders connected via cylinder ducts to communicationorifices situated in a communication face of the cylinder block that isperpendicular to the axis of rotation, pistons mounted to slide in thecylinders being suitable for co-operating with the cam, which cam isprovided with a plurality of lobes, each of which has two ramps, themotor further comprising a fluid distributor having a distribution facethat is perpendicular to the axis of rotation and that is suitable forbeing in abutment against the communication face of the cylinder block,said distribution face being provided with distribution orificescomprising orifices suitable for being connected to a fluid feed andorifices suitable for being connected to a fluid discharge, the fluiddistributor being constrained to rotate with the cam, so that there isone ramp of the cam that corresponds to each distribution orifice, saiddistribution orifices being suitable for communicating one after anotherwith the communication orifices while the cylinder block and thedistributor are turning relative to each other, an edge of each of atleast certain communication orifices being provided with at least onenotch suitable for establishing a small section of communication with adistribution orifice.
 12. A motor according to claim 11, in which theedge of each communication orifice has a leading portion via whichcommunication between the communication orifice and the distributionorifices opens while the cylinder block and the distributor are turningrelative to each other in a given direction of relative rotation, and atrailing portion via which communication between the communicationorifice and the distribution orifices closes while the cylinder blockand the distributor are turning relative to each other in said directionof relative rotation, each leading portion and each trailing portion ofthe edges of at least certain communication orifices having a notchsuitable for establishing a small section of communication with adistribution orifice.
 13. A motor according to claim 12, wherein thenotches in the leading portion and in the trailing portion of the edgeof each of said communication orifices are symmetrical.
 14. A hydraulicmotor according to claim 12, having two directions of rotation, one ofwhich directions being a preferred direction, wherein those portions ofthe edges of said communication orifices which, in the preferredoperating direction, constitute respectively the leading portions andthe trailing portions are provided respectively with large notches andwith small notches.
 15. A hydraulic motor according to claim 11, whereinthe edge of each communication orifice has a leading portion via whichcommunication between the communication orifice opens while the cylinderblock and the distributor are rotating relative to each other in a givendirection of relative rotation, and a trailing portion via whichcommunication between the communication orifice and the distributionorifices closes while the cylinder block and the distributor arerotating relative to each other in the same direction of relativerotation, and wherein, for at least certain communication orifices, atleast the leading portion has a shape that is substantiallycomplementary to a shape of the edges of the distribution orifices viawhich the communication between the communication orifices and thedistribution orifices open.
 16. A motor according to claim 15, whereinthe distribution orifices are substantially circular, and wherein, foreach of at least some of the communication orifices, the leading portionof the orifice has a convex shape as seen from the inside of theorifice.
 17. A motor according to claim 16, wherein for at least some ofthe communication orifices, the leading portion and the trailing portionare substantially convex, as seen from inside the orifice.
 18. A motoraccording to claim 11, wherein each ramp of the cam has a convex portionand a concave portion, two adjacent ramps being connected togethereither via a cam crest zone extending between their respective convexregions, or via a cam trough zone extending between their respectiveconcave regions, wherein said cam crest zone and said cam trough zoneare substantially circular arcs centered on the axis of rotation, sothat when the pistons are co-operating with said zones, radial strokesof said pistons are substantially zero, and wherein the distributionorifices and the communication orifices have dimensions such that, whilethe cylinder block and the distributor are rotating relative to eachother, each distribution orifice remains momentarily isolated from anycommunication orifice.
 19. A motor according to claim 11, having twoactive operating cubic capacities, namely a large cubic capacity inwhich all of the pistons are active, and a small cubic capacity in whichonly certain pistons are active, wherein only each of the communicationorifices of the cylinders of the pistons that are active in the smallcubic capacity is provided with a notch in its edge.
 20. A motoraccording to claim 11, having two active operating cubic capacities,namely a large cubic capacity in which all of the pistons are active,and a small cubic capacity in which only certain pistons are active,wherein only the edges of all of the communication orifices have atleast one notch each, and wherein the edges of the communicationorifices of the cylinders of the pistons that are active in the smallcubic capacity have notches that are larger than the notches in theedges of the communication orifices of the cylinders of the pistons thatare inactive in the small cubic capacity.